1. Field of the Invention
The invention relates to a process for the continuous preparation of aromatic amines by hydrogenation of the corresponding nitroaromatics in the presence of catalysts arranged in reaction spaces, in which an adiabatically operated reaction space RA is connected downstream of an isothermally operated reaction space RI and RA additionally also has a separate feed for the nitroaromatic to be hydrogenated, RI is fed with the nitroaromatic to be hydrogenated from the start to the end of the hydrogenation, and the product mixture emerging from RI is fed into RA from the start to the end of the hydrogenation, wherein RA additionally is fed via the separate feed with the nitroaromatic to be hydrogenated.
2. Description of Related Art
Aromatic amines are important intermediate products which must be prepared inexpensively and in large amounts. Production installations for aromatic amines are therefore as a rule built for very high capacities. The hydrogenation of nitroaromatics is a highly exothermic reaction. The removal of the heat of reaction and its use for energy are therefore an important point in the preparation of nitroaromatics.
DE-OS-28 49 002 describes a process for the reduction of nitro compounds in the presence of palladium-containing multi-component supported catalysts of fixed position in cooled tube bundle reactors. The catalysts essentially comprise 1 g to 20 g of palladium, 1 g to 20 g of vanadium and 1 g to 20 g of lead per liter of α-Al2O3. A disadvantage of the gas phase hydrogenations described in the patent literature mentioned is the low specific loading of the catalysts with the nitroaromatic to be hydrogenated. The loadings stated are only approx. 0.4 kgnitroaromatic/(1catalyst.h) to 0.5 kgnitroaromatic/(1catalyst.h). The loading in this context is defined as the amount of nitroaromatic in kg which is passed over the catalyst per liter of bulk catalyst (catalyst volumes here and in the following relate to the bulk volume) within one hour. An unsatisfactory space-time yield is associated with the low catalyst loading in large-scale industrial processes for the preparation of aromatic amines. The selectivities at the start of an operating period are furthermore significantly lower than towards the end, which leads to losses in yield and problems in the working up of the crude product.
In the process variant described in GB 1 452 466, the hydrogenation of nitroaromatics in thermostatically controlled tube bundle reactors is supplemented by a downstream adiabatically (i.e. without thermostatic control) operated reactor. Supported copper or palladium catalysts, inter alia, are employed as catalysts. In this process, the thermostatically controlled (i.e. isothermally operated) reactor and the adiabatically operated reactor, which can also be arranged in one apparatus, are connected in series, i.e. the product mixture emerging from the thermostatically controlled reactor is the educt mixture for the adiabatically operated reactor. The possibility of additionally charging the adiabatically operated reactor with nitrobenzene via a separate feed is not disclosed in this specification. In preferred embodiments, an incomplete conversion (e.g. only 70%) in the isothermally operated part is consciously accepted.
In EP 1 524 259 A1 inter alia a 2-stage process for the preparation of aromatic amines is described, in which the second process stage serves to bring the conversion to completion. In this context, an adiabatically operated reaction which contains a catalytically coated monolith as the catalyst is employed. By using this “secondary reactor” in the second stage, the service life of the “main reactor” (the first stage) can be prolonged, since complete conversion can still be achieved with the aid of the secondary reactor when the nitroaromatic has already broken through in the main reactor.
A disadvantage of the abovementioned processes is that the secondary reactor only converts the nitroaromatic into the aromatic amine if the conversion of the main reactor is not (any longer) complete. If the longest possible as complete as possible conversion in the main reactor is sought (EP 1 524 259 A1), this means that the secondary reactor remains unused for large parts of the operating time (namely the complete conversion phase of the main reactor). This leads to breakdown of valuable products by secondary reactions, which are catalyzed by the catalyst contained in the secondary reactor. If incomplete conversion is consciously accepted in the main reactor (certain embodiments of GB 1 452 466), this means that the main reactor must be overloaded, i.e. charged with more nitroaromatic than can be reacted, from the beginning. This leads to rapid deactivation of and damage to the catalyst, so that the service life is reduced and the advantage of the secondary reactor may be overcompensated. However, in the first case also, in which the main reactor is initially operated with complete conversion, at the end of the running time very high contents of unreacted nitroaromatic in the product stream of the main reactor and therefore an overloading of the secondary reactor, the catalyst of which is then rapidly deactivated, rapidly occur.